1. Field of the Invention
The present invention relates to a concentration measuring method and apparatus for an absorption component in a scattering medium. More specifically, the present invention relates to a concentration measuring method and apparatus for an absorption component in a scattering medium, in which at least two light rays with predetermined wavelengths, whose scattering coefficients are different and have a known ratio, are made incident on a scattering medium such as living bodies having various shapes, light which has a predetermined wavelength and diffuses and propagates inside the scattering medium and comes to the surface is detected to obtain the light intensity and mean flight pathlength (mean optical pathlength) at the detection position, and on the basis of the light intensity and mean flight pathlength, a relative value or an absolute value of concentration of a specific absorption component in the scattering medium, oxygen saturation of hemoglobin, and a change in time or a spatial distribution thereof can be highly accurately and noninvasively measured without any influence of the shape of the scattering medium.
2. Related Background Art
There is a strong demand for highly accurate and noninvasive measurement of a relative value and an absolute value of concentration of a specific absorption component in a scattering medium such as a living body, and a change in time as well as a spatial distribution thereof. Various methods or examinations have been used or made so far, including a method using continuous light (CW light) or modulated light (e.g., pulse light, rectangular waveform light, or sine-wave-modulated light) and a method using light components with different wavelengths.
In these conventional techniques, a method and apparatus for sufficiently accurately measuring the concentrationof a specific absorption component in an object such as living bodies whose regions have different shapes or shapes have individual differences even in identical regions have not been developed yet. This poses a serious problem in noninvasive measurement of a living body using light, and a strong demand has arisen for improvement thereof.
Light incident on a scattering medium such as a living body diffuses and propagates inside the scattering medium while being scattered and absorbed, and partially comes to the surface. Since the scattering medium is normally surrounded by air, the light coming to the surface dissipates through the free space.
In the measurement of internal information of a scattering medium, light that has come to the surface in the above way is detected. In this case, if the boundary condition (shape) of the scattering medium changes, e.g., depending on whether the scattering medium has a spherical shape or a rectangular parallelopiped shape, the intensity and behavior of light coming to a predetermined position of the surface changes greatly.
Hence, to increase the accuracy of such measurement, the behavior of light in the scattering medium must be understood well. As is recently known, the behavior of light in a scattering medium can be relatively accurately described and analyzed by analysis, experiments, and examinations of Monte Carlo simulation using a computer, or photon diffusion theory.
As described above, to understand the behavior of light in a scattering medium, Monte Carlo simulation or photon diffusion theory is conventionally used. However, Monte Carlo simulation takes a verylong time for calculation and cannot calculate the concentration of a specific absorption component in a scattering medium from the result of simulation.
To use the photon diffusion theory, a boundary condition must be set to actually solve a photon diffusion equation. However, the boundary condition largely depends on the shape of a scattering medium. For this reason, for accurate measurement, a new boundary condition must be set to solve a photon diffusion equation every time the shape of the scattering medium changes. Additionally, a relatively accurate boundary condition can be set for only a scattering medium with a very simple shape, such as an infinite space, semi-infinite space, infinite circular cylinder, or infinitely spreading slab having a limited thickness. As a result, to measure a living body having a complex shape using the photon diffusion theory, it is indispensable to use an approximate boundary condition, resulting in a large measurement error.
As a solution to these problems, the present inventor has already developed and filed a patent application (Japanese Patent Application Laid-Open Gazette No. Hei 8-94517) for a method of measuring the absorption coefficient of a scattering material and the concentration of an absorber on the basis of the Micro-Beer-Lambert law.
The method in Japanese Patent Application Laid-Open Gazette No. Hei 8-94517 is excellent because it can quantitatively measure an absorption coefficient independently of the boundary condition (shape) of an object to be measured. However, this method uses a plurality of light components with different wavelengths whose scattering characteristics are equal or can be regarded to be equal for a scattering medium to be measured. Hence, the method disclosed in Japanese Patent Application Laid-Open Gazette No. Hei 8-94517 is not satisfactory because it can use only limited wavelengths, and as the difference in scattering characteristics between the plurality of wavelengths of light components in use increases, the measurement error increases, or if the difference further increases, measurement is disabled.
As described above, a diffused light handling method which can be systematically applied, without any limitation on usable wavelengths, to a scattering medium having scattering characteristics depending on a wavelength and a different boundary condition has not been developed yet. For this reason, it is conventionally impossible to systematically accurately measure the concentration of an internal specific absorption component in such a scattering medium without limiting a wavelength to be used.
The present invention has been made to solve the above problems of the prior art, and has as its object to provide a concentration measuring method and apparatus for an absorption component in a scattering medium, in which the basic relationship associated with the behavior of light in a scattering medium having scattering characteristics depending on a wavelength and having a different boundary condition is newly disclosed, even when the scattering characteristics depend on the wavelength, a relative value or an absolute value of concentration of a specific absorption component in scattering media having various shapes can be accurately measured using that relationship without any limitation on the wavelength to be used and any influence of the wavelength dependence of such scattering characteristics, and a change in time or a spatial distribution thereof can also be accurately measured without any influence of the wavelength dependence of the scattering characteristics.
In the present invention, at least two light rays having predetermined wavelengths and a known ratio of transport scattering coefficients are made incident on a scattering medium having various boundary conditions (shapes), a light intensity and mean flight pathlength of each light ray having the predetermined wavelength at the light detection position are obtained, and on the basis of these values, a relative value or an absolute value of concentration of a specific absorption component are obtained by arithmetic processing without any influence of the boundary condition of the scattering medium or wavelength dependence of scattering characteristics.
Specifically, a concentration measuring method for an absorption component in a scattering medium according to the present invention comprises
(a) a light generation step of generating at least two light rays having predetermined wavelengths (light rays having wavelengths of two types or more), the light rays having different transport scattering coefficients for a scattering medium as an object to be measured, and a known ratio of the transport scattering coefficients,
(b) a light incidence step of making the light rays incident from a light incident position into the scattering medium,
(c) a photodetection step of detecting the light ray which has propagated inside the scattering medium at at least one photodetection position different from the-light incident position to acquire at least one photodetection signal,
(d) a parameter detection step of detecting, on the basis of the photodetection signal, a light intensity and a mean flight pathlength at the light detection position for each of the at least two light rays having predetermined wavelengths, and
(e) an arithmetic processing step of calculating a concentration of an absorption component on the basis of a predetermined relationship between the ratio of the transport scattering coefficients, the light intensity, the mean flight pathlength, and a difference between absorption coefficients per unit concentration of the absorption component for the at least two light rays having predetermined wavelengths.
A concentration measuring apparatus for an absorption component in a scattering medium according to the present invention comprises
(a) a light source for generating at least two light rays having predetermined wavelengths (light rays having wavelengths of two types or more), the light rays having different transport scattering coefficients for a scattering medium as an object to be measured, and a known ratio of the transport scattering coefficients,
(b) light incidence means for making the light rays incident from a light incident position into the scattering medium,
(c) photodetection means for detecting the light ray which has propagated inside the scattering medium at at least one photodetection position different from the light incident position to acquire at least one photodetection signal,
(d) parameter detection means for detecting, on the basis of the photodetection signal, a light intensity and a mean flight pathlength at the light detection position for each of the at least two light rays having predetermined wavelengths, and
(e) arithmetic processing means for calculating a concentration of an absorption component on the basis of a predetermined relationship between the ratio of the transport scattering coefficients, the light intensity, the mean flight pathlength, and a difference between absorption coefficients per unit concentration of the absorption component for the at least two light rays having predetermined wavelengths.
In the method and apparatus of the present invention, each of the at least two light rays having predetermined wavelengths may be a pulse light ray.
Each of the at least two light rays having predetermined wavelengths may be a sine-wave-modulated light ray having a predetermined modulation frequency component, the light intensity may be calculated from (i) a DC component of the photodetection signal or (ii) an amplitude of a signal having the predetermined modulation frequency component, which is contained in the photodetection signal, and the mean flight pathlength may be calculated from a phase delay of the signal having the predetermined modulation frequency component.
Each of the at least two light rays having predetermined wavelengths may be a modulated light ray having a predetermined repetitive modulation frequency component, the light intensity may be calculated from (i) a DC component of the photodetection signal or (ii) an amplitude of a signal having the predetermined repetitive modulation frequency component or a frequency component of an integer multiple thereof, which is contained in the photodetection signal, and the mean flight pathlength may be calculated from a phase delay of the signal having the predetermined repetitive modulation frequency component or the frequency component of an integer multiple thereof.
The predetermined relationship between the ratio of the transport scattering coefficients, the light intensity, the mean flight pathlength, and the difference between the absorption coefficients per unit concentration of the absorption component for the at least two light rays having predetermined wavelengths is preferably a relationship derived on the basis of a fact that a value obtained by partially differentiating a natural logarithm of the detected light intensity by the absorption coefficient equals the mean flight pathlength without neglecting a difference in mean flight pathlength due to the difference in scattering coefficient.
In the method and apparatus according to one aspect of the present invention, in the arithmetic processing step (arithmetic processing means), the concentration of the absorption component in the scattering medium is preferably calculated on the basis of a relationship represented by                     V        =                  xe2x80x83                ⁢                  ln          ⁢                                                                                          I                    1                                    ⁡                                      (                                          λ                      1                                        )                                                  ⁢                                  B                  2                                                                                                  kI                    1                                    ⁡                                      (                                          λ                      2                                        )                                                  ⁢                                  B                  1                                                      [                                          (                                                      ϵ                    2                                    -                                      ϵ                    1                                                  )                            ⁢                              xe2x80x83                            ⁢                              {                                                      p                    ⁢                                          ⟨                                                                        L                          1                                                ⁡                                                  (                                                      λ                            2                                                    )                                                                    ⟩                                                        +                                                                                                                  xe2x80x83                    ⁢                                    (                              1                -                p                            )                        ⁢                          k                        ⁢                          ⟨                                                L                  1                                ⁡                                  (                                      λ                    1                                    )                                            ⟩                                }                -                                                      xe2x80x83                    ⁢                                                    ϵ                1                            ⁡                              (                                  1                  -                                      k                                                  )                                      ⁢                          ⟨                                                L                  1                                ⁡                                  (                                      λ                    1                                    )                                            ⟩                                ]                          -          1                    
where
V is the concentration of the absorption component,
xcex51 is an absorption coefficient per unit concentration of the absorption component for light having a wavelength xcex1,
xcex52 is an absorption coefficient per unit concentration of the absorption component for light having a wavelength xcex52,
 less than L1(xcex1) greater than  is a mean flight pathlength for the light having the wavelength xcex1,
 less than L1(xcex2) greater than  is a mean flight pathlength for the light having the wavelength A2,
I1(xcex1) is a detected light intensity for light having an incident light intensity B1 and the wavelength xcex1,
I1(xcex2) is a detected light intensity for light having an incident light intensity B2 and the wavelength xcex2,
k is a ratio (xcexcs2xe2x80x2/xcexcs1xe2x80x2) of a transport scattering coefficient xcexcs2xe2x80x2 for the light having the wavelength xcex2 to a transport scattering coefficient xcexcs1xe2x80x2 for the light having the wavelength xcex1, and
p is a predetermined value satisfying 0xe2x89xa6pxe2x89xa61.
In the method and apparatus according to another aspect of the present invention, in the arithmetic processing step (arithmetic processing means), the concentration of the absorption component in the scattering medium is preferably calculated on the basis of a relationship represented by                     V        =                  xe2x80x83                ⁢                  ln          ⁢                      xe2x80x83                    ⁢                                                    I                2                            ⁢                              xe2x80x83                            ⁢                              (                                  λ                  1                                )                            ⁢                              xe2x80x83                            ⁢                              I                1                            ⁢                              xe2x80x83                            ⁢                              (                                  λ                  2                                )                                                                    I                2                            ⁢                              xe2x80x83                            ⁢                              (                                  λ                  2                                )                            ⁢                              xe2x80x83                            ⁢                              I                1                            ⁢                              xe2x80x83                            ⁢                              (                                  λ                  1                                )                                              xc3x97                      [                                          (                                                      ϵ                    2                                    -                                      ϵ                    1                                                  )                            ⁢                              xe2x80x83                            ⁢                              {                                                      p                    ⁢                                          ⟨                                                                        L                          2                                                ⁡                                                  (                                                      λ                            2                                                    )                                                                    ⟩                                                        +                                                                                                                  xe2x80x83                    ⁢                                                    (                                  1                  -                  p                                )                            ⁢                              k                            ⁢                              ⟨                                                      L                    2                                    ⁡                                      (                                          λ                      1                                        )                                                  ⟩                                      }                          -                              (                                          ϵ                2                            -                              ϵ                1                                      )                    ⁢                      xe2x80x83                    ⁢                      {                                          q                ⁢                                  ⟨                                                            L                      1                                        ⁡                                          (                                              λ                        2                                            )                                                        ⟩                                            +                                                                                    xe2x80x83                    ⁢                                                    (                                  1                  -                  q                                )                            ⁢                              k                            ⁢                              ⟨                                                      L                    1                                    ⁡                                      (                                          λ                      1                                        )                                                  ⟩                                      }                          -                                            ϵ              1                        ⁡                          (                              1                -                                  k                                            )                                ⁢                      {                                          ⟨                                                      L                    2                                    ⁡                                      (                                          λ                      1                                        )                                                  ⟩                            +                                                                                                  xe2x80x83                        ⁢                          ⟨                                                L                  1                                ⁡                                  (                                      λ                    1                                    )                                            ⟩                        }                    ]                          -          1                    
where
V is the concentration of the absorption component,
xcex51 is an absorption coefficient per unit concentration of the absorption component for light having a wavelength xcex1,
xcex52 is an absorption coefficient per unit concentration of the absorption component for light having a wavelength xcex2,
 less than L1(xcex1) greater than  is a mean flight pathlength at a photodetection position r1 for the light having the wavelength xcex1,
 less than L1(xcex2) greater than  is a mean flight pathlength at the photodetection position r1 for the light having the wavelength xcex2,
 less than L2(xcex1) greater than  is a mean flight pathlength at a photodetection position r2 for the light having the wavelength xcex1,
 less than L2(xcex2) greater than  is a mean flight pathlength at the photodetection position r2 for the light having the wavelength xcex2,
I1(xcex1) is a detected light intensity at the photodetection position r1 for light having an incident light intensity B1 and the wavelength xcex1,
I1(xcex2) is a detected light intensity at the photodetection position r1 for light having an incident light intensity B2 and the wavelength xcex2,
I2(xcex1) is a detected light intensity at the photodetection position r2 for the light having the incident light intensity B1 and the wavelength xcex1,
I2(xcex2) is a detected light intensity at the photodetection position r2 for the light having the incident light intensity B2 and the wavelength xcex2,
k is a ratio (xcexcs2xe2x80x2/xcexcs1xe2x80x2) of a transport scattering coefficient xcexcs2xe2x80x2 for the light having the wavelength xcex2 to a transport scattering coefficient xcexcs1xe2x80x2 for the light having the wavelength xcex1,
p is a predetermined value satisfying 0xe2x89xa6pxe2x89xa61, and
q is a predetermined value satisfying 0xe2x89xa6qxe2x89xa61.
In the present invention, xe2x80x9ctransport scattering coefficients are different from each otherxe2x80x9d means that the difference between the transport scattering coefficients for each of at least two light rays having predetermined wavelengths for the scattering medium as a target measurement object is so large that it cannot be neglected.
In the present invention, the concentration of a specific absorption component is calculated on the basis of the basic relationship that holds for various scattering media having scattering characteristics depending on wavelengths and different boundary conditions, i.e., the relationship between the light intensity at the photodetection position, mean flight pathlength, and the difference or ratio between the absorption coefficients per unit concentration of the absorption component for the at least two light rays having predetermined wavelengths, and the value or ratio of the transport scattering coefficients of the scattering component for the at least two light rays having predetermined wavelengths. For this reason, the concentration of the specific absorption component can be more accurately measured without any limitation on the wavelength to be used and any influence of the boundary condition (shape) of the scattering medium. In addition, the change in time or spatial distribution of the concentration of the specific absorption component can also be measured.
In the present invention, the light intensity and mean flight pathlength obtained from actual measurement values are used as parameters for arithmetic processing of the concentration of the specific absorption component. These parameters are obtained using almost all light beams obtained at the photodetection position, i.e., have integrated values. Hence, a high signal-to-noise ratio can be obtained, and consequently, high measurement accuracy can be obtained.
In the present invention, measurement is performed by making at least two light rays having predetermined wavelengths, which have different transport scattering coefficients and a known ratio thereof, incident on a scattering medium. The difference between absorption coefficients of an absorption component for a predetermined wavelength is obtained from measurement values, and the concentration of a specific absorption component is obtained from this difference.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.